Inline list validation silently breaks past the 255-character formula limit

Cluster: data-validation

Scenario

A user attaches a list-type data validation and supplies the allowed options as an inline comma-separated formula string, say formulae: ['"opt1,opt2,opt3,…"']. The spreadsheet format enforces a hard limit of 255 characters on the inline drop-down formula string for a single cell. When the joined string exceeds 255 characters, the written file is structurally valid but the application silently refuses to show the drop-down: the cell has a validation attached, yet no visible list. Because nothing warns the author, the failure looks like a library bug when it is actually an application-format constraint.

The known-good alternative has no length limit: move the options into a range, on any sheet, and reference it, as formulae: ['=Sheet2!$A$2:$A$21'] or formulae: ['$D$5:$F$5'].

Spec note, not a corpus case: the written output is structurally valid, so there is no malformed serialization to assert against. The defect is a silent usability cliff plus an undecided design choice between warning, throwing and documenting. The 255-char fact and the warn-on-overflow requirement are durable product guidance.

Desired behavior

• Do not let an author cross the limit silently. When an inline list-validation formula string would exceed the 255-character limit, the library surfaces it, at minimum a clear diagnostic and under a strict or validate mode a thrown error, rather than emitting a validation that opens as an invisible no-op.
• Point at the fix. The diagnostic names the working alternative: move the options to a range and reference it, cross-sheet or same-sheet, which the format allows without a length cap.
• The range-reference form is first-class and lossless. A list validation whose source is a range reference round-trips faithfully, already locked by corpus cases, so the recommended escape hatch is solid.
• The 255-char limit applies to the inline literal form only; range and defined-name sources are unbounded and are the right tool for long option sets.

Open questions

• Warn or throw: is a console diagnostic enough for the default path, with a strict mode that throws, or should authoring an over-limit inline list always fail fast?
• Should the library auto-spill an over-limit inline list into a hidden helper range and reference it automatically, or is that too much implicit magic, since it adds a sheet or range the author did not ask for?
• Is the 255 count measured in UTF-16 code units, code points, or bytes for non-ASCII options, and does the target application count the surrounding quotes and separators?

Related: multiselect-dropdown-validation, cross-sheet-list-validation-x14, list-validation-value-source-forms-roundtrip, list-validation-defined-name-and-cross-sheet-range-source, whole-column-data-validation-bounded-memory.